1. Field of the Invention.
This invention relates to determination of blood electrolytes, and more particularly relates to a permeable membrane prepared from a crosslinked polyetherurethane composition and a blood electrolyte sensing device including the membrane.
2. Background of the Invention.
Thermoplastic polyurethanes to be used as elastomers and the like have been known for a long time. Products prepared from organic compounds having two or more isocyanate groups, high molecular weight polyetherglycols, and low molecular weight diols and diamines as chain extenders are conventionally referred to as polyetherurethanes, and this term, abbreviated PEU, will be used in this disclosure for polyurethanes having a polyether backbone.
PEU compositions develop microdomains conventionally termed hard segments and soft segments, and as a result are often referred to as segmented PEUs. The hard segments form by localization of the portions of the polymer molecules which include the isocyanate and extender components and are generally of high crystallinity. The soft segments form from the polyether glycol portions of the polymer chains and generally are either noncrystalline or of low crystallinity.
PEU formulations having properties such as water absorption and mechanical strength which make them useful for specific applications have been developed. It is also known that these properties are greatly influenced by the choice or ratio of the components of the formulations. For example, Szycher, in U.S. Pat. No. 4,131,604 discloses a PEU molded into a bladder capable of continuous flexing making it useful in heart assist systems. In order to achieve the desired properties, the polyetherglycol component of the composition is limited to polytetramethylene glycol. Alberino et al., in U.S. Pat. No. 4,321,333, discloses that, by using blends of diisocyanates, formulations having significantly improved green strength are obtained. Quiring et al., in U.S. Pat. No. 4,371,684, reports that thermoplastic polyurethanes of improved extrudability are obtained by use of two low molecular weight diol chain extenders instead of the theretofore conventional butanediol.
Lyman et al. (Journal of Biomedical Materials Research 1 17 (1967) discloses dialysis membranes prepared from uncrosslinked PEU by solution casting onto a glass plate.
Sensing devices for determination of blood components are well-known. All such devices utilize a membrane which is permeable to the blood component being analyzed. U.S. Pat. Nos. 4,534,356 and 4,536,274, to Papadakis disclose electrochemical sensors in which membranes useful for blood gas analysis are broadly defined as hydrogels or hydrophilic polymers or copolymers and membranes useful for blood pH determinations are copolymers of fluorine-containing monomers.
A portable assembly for analysis of blood oxygen and carbon dioxide which includes a blood sampler, an electrochemical sensor and blood gas analyzer is disclosed by Kronenberg et al. in U.S. Pat. No. 4,615,340. The sensor includes a gas permeable, ion permeable membrane fabricated of polycarbonate or cellulose and a gas permeable, ion impermeable membrane of polytetrafluoroethylene or polypropylene.
Blood gases are measured by Lubbers et al. in U.S. Pat. No. Re. 31,879 by a fluorescence-based sensor using selective gas permeable membranes and optical fibers to direct incident light to a dye and fluorescence from the dye.
A fiber optic pH probe for physiological studies using an ion permeable cellulose membrane is described by Peterson et al. in U.S. Pat. No. 4,200,110.
Baxter, in U.S. Pat. No. 4,505,799, discloses an ion sensitive field effect transistor (ISFET) for measurement of hydrogen ions which includes a membrane which may be silicon nitride or aluminum oxide.
Potter, in U.S. Pat. No. 4,534,355, shows an electrochemical device for sensing blood oxygen and carbon dioxide having a linear PEU membrane coated onto the mounting of the device. The Potter membrane is disclosed to absorb up to 50% water. On the other hand, Korlatski, in U.S. Pat. No. 4,123,589, describes a PEU membrane which exhibits impermeability to water making it useful as a food casing.
Ionic permeability across a hydrophilic PEU membrane occurs by partitioning ions in a fluid between absorbed water in the membrane and the fluid. Thus, the rate at which an ionic solute crosses a membrane depends on the water content of the membrane i.e., faster transmembrane passage and shorter analysis time may be achieved with membranes of high water absorptivity. The present invention is directed to membranes of exceptionally high water retention which, in addition, retain the mechanical strength necessary for use in blood electrolyte sensing devices.